1. Field of the Invention
The present invention relates to an image scanner, and more particularly relates to an image scanner having a mechanism by which the movement of a carrier can be stabilized so as to prevent the deviation of the carrier.
2. Related Art
An image scanner reads out image information such as figures and characters expressed on a document placed on a platen while the document is optically scanned. Concerning the mechanism of the image scanner, there are two types of image scanners. One is a one-carrier type image scanner in which an optical system composed of a light source (lamp), mirror and lens, CCD and drive circuit are mounted on one carrier, and this carrier is moved in parallel to a document surface at a scanning speed. The other is a two-carrier type image scanner in which a lens and CCD reading circuit are fixed to a frame of the image scanner, and its optical system composed of a light source (lamp) and mirror is divided into two and mounted on two different carriers, and the first carrier is moved in parallel to the document surface at a scanning speed and the second carrier is moved at half the scanning speed of, but in the same direction as, the first carrier.
The two-carrier type image scanner is advantageous in that: the number of parts to be mounted on each carrier is small; the construction is simple; the image scanner is Light in weight; and the response of each carrier is high. However, it is difficult to accurately assemble and adjust the two carriers in order to move them along the optical axis so that the length of the optical axis can be maintained constant at all times. Further, since the two carriers must be driven at different speeds, the construction of the drive system becomes complicated. Furthermore, when each carrier is accelerated or decelerated, the formed image tends to be blurred by the deviation of each carrier caused when it is driven.
On the other hand, in the one-carrier type image scanner, the optical system including the light source, mirror and lens, and the CCD are mounted on one carrier, and this carrier must be moved in parallel to the document surface at the scanning speed. Accordingly, the construction of the carrier becomes complicated, the weight is increased, the response is low, and it takes time to accelerate the carrier to a predetermined speed. However, in this case, there is only one carrier. Therefore, this carrier is advantageous in that the adjustment of the optical passage length and also adjustment of the optical axis may be conducted on a single carrier.
The one-carrier type and two-carrier the image scanners have the following common problem. A skew of the carrier with respect to the image surface, that is, an inclination or deviation of the carrier, must be adjusted.